As is well known, valve timing has a significant effect on engine performance and the optimum setting varies with engine operating conditions. To optimize performance under different operating conditions, it is necessary to be able to vary the valve timing.
Various variable valve timing mechanisms have been proposed in the past that achieve a variable phase. These mechanisms have suffered from various problems. Some, though feasible, have been costly to implement and some have developed excessive friction or not proved to be reliable. Furthermore, many could not be fitted as a modification to existing engines and required much of the valve train and cylinder head to be redesigned.
The most relevant prior art is believed to be the Applicants' own earlier proposal in EP-A-0 733 154. This discloses a valve operating mechanism comprising a hollow shaft, a sleeve journalled on the hollow shaft and fast in rotation with a cam, a coupling yoke connected by a first pivot pin to the hollow shaft and by a second pivot pin to the sleeve and means for moving the yoke radially to effect a phase change between the hollow shaft and the sleeve, wherein the means for moving the yoke radially comprise an actuating rod slidably received in the hollow shaft, a cam surface on the actuating rod and a plunger passing through a generally radial bore in the hollow sleeve to cause the yoke to move radially in response to axial movement of the actuating rod.